The internalization of extracellular material into cells is commonly performed by a process called endocytosis (Rabinovitch, 1995, Trends Cell Biol. 5(3):85-7; Silverstein, 1995, Trends Cell Biol. 5(3):141-2; Swanson et al., 1995, Trends Cell Biol. 5(3):89-93; Allen et al., 1996, J. Exp. Med. 184(2):627-37). Endocytosis may fall into two general categories: phagocytosis, which involves the uptake of particles, and pinocytosis, which involves the uptake of fluid and solutes.
Phagocytosis is conducted primarily by highly specialized cells, such as macrophages, monocytes and neutrophils, with the goal of clearing pathogens and/or debris (Aderem et al., 1999, Annu. Rev. Immunol. 17:593-623; Stuart et al., 2008, Nat. Rev. Immunol. 8(2):131-41). Phagocytosis is an active and highly regulated process that is dependent on both actin (Wang et al., 1984, J. Cell Biol. 98(4):1328-41; Greenberg et al., 1990, J. Exp. Med. 172(6):1853-6) and myosin (Stendahl et al., 1980, J. Cell Biol. 84(2):215-24; Valerius et al., 1981, Cell 24(1):195-202; Diakonova et al., 2002, Mol. Biol. Cell 13(2):402-11), but is typically independent of clathrin (Aderem et al., 1999, Annu. Rev. Immunol. 17:593-623; Tse et al., 2003, J. Biol. Chem. 278(5):3331-8). Phagocytosis involves a highly efficient mechanism for internalizing foreign invaders and particles that the phagocytic cell recognizes through an array of receptors (Silverstein, 1995, Trends Cell Biol. 5(3):141-2). These specific cell-surface receptors recognize opsonin, including Fc receptors (FcR) on macrophages that are activated by antibodies bound to the targets (Greenberg et al., 1993, J. Exp. Med. 177(2):529-34; Ravetch, 1994, Cell 78(4):553-60). A number of cytoskeletal proteins have been specifically shown to be involved in FcR phagocytosis, such as myosins (Cox et al., 2002, Nat. Cell Biol. 4(7):469-77; Diakonova et al., 2002, Mol. Biol. Cell 13(2):402-11).
A phagocytic cell engulfs another cell or particle that is IgG-opsonized in a coordinated process of adhesion, pseudopod extension, and internalization with phagosome closure. Upon initial binding of IgG, the phagocyte's Fc receptors (FcγRs) activate cytoskeletal assembly with rapid accumulation of phosphopaxillin (Greenberg et al., 1994, J. Biol. Chem. 269(5):3897-3902; Allen et al., 1996, J. Exp. Med. 184(2):627-37) and F-actin (Wang et al., 1984, J. Cell Biol. 98(4):1328-41; Greenberg et al., 1991, J. Cell Biol. 113:757-767) among other components at a “phagocytic synapse.” Non-muscle myosins also accumulate and suggest a role(s) for contractile motors during particle internalization (Stendahl et al., 1980, J. Cell Biol. 84(2):215-24; Valerius et al., 1981, Cell 24(1):195-202; Diakonova et al., 2002, Mol. Biol. Cell 13(2):402-11). Signaling activities that influence synapse assembly continue to be clarified (Aderem et al., 1999, Annu Rev. Immunol. 17:593-623) and are presumably key to how the macrophage distinguishes foreign cells or particles from autologous cells of “self.” Autologous cells are believed to be opsonized by Ig (Turrini et al., 1993, Blood 81(11):3146-52), and so activation differences only tell a portion of the story.
In addition to activation, professional phagocytes have been shown to differentiate from non-self and self, based on studies with knockout mice lacking the membrane receptor CD47 (Oldenborg et al., 2000, Science 288(5473):2051-4). CD47 is a ubiquitous member of the Ig superfamily that interacts with the immune inhibitory receptor SIRPα (signal regulatory protein) found on macrophages (Fujioka et al., 1996, Mol. Cell. Biol. 16(12):6887-99; Veillette et al., 1998, J. Biol. Chem. 273(35):22719-28; Jiang et al., 1999, J. Biol. Chem. 274(2):559-62). Although CD47-SIRPα interactions appear to deactivate autologous macrophages in mouse, severe reductions of CD47 (perhaps 90%) are found on human blood cells from some Rh genotypes that show little to no evidence of anemia (Mouro-Chanteloup et al., 2003, Blood 101(1):338-344) and also little to no evidence of enhanced cell interactions with phagocytic monocytes (Arndt et al., 2004, Br. J. Haematol. 125(3):412-4). This interaction activates SHP-1 phosphatase (Tsuda et al., 1998, J. Biol. Chem. 273(21):13223-9; Veillette et al., 1998, J. Biol. Chem. 273(35):22719-28; Vernon-Wilson et al., 2000, Eur. J. Immunol. 30(8):2130-7; Kant et al., 2002, Blood 100(5):1852-1859) which leads to downstream targets such as non-muscle myosin IIA (NMMIIA) (Tsai et al., 2008, J. Cell Biol. 180(5):989-1003).
Macrophages are proficient at discriminating between foreign cells or particles in the microbe size range. However, this self signal, CD47, is not understood in targets less than 500 nm in size. Particles that are in the nanometer length scales may undergo a different process from the traditional phagocytosis pathways (Wright et al., 1984, Nature 309(5966):359-61) which may include macropinocytosis (Koval et al., 1998, Exp. Cell. Res. 242(1):265-73; Rejman et al., 2004, Biochem. J. 377(Pt 1):159-69). Thus, macrophage targets using the CD47 self signal have not been shown in the nanoscale range, making the hypothesis of the viability of CD47 inhibition unknown at these smaller length scales.
Thus, there is a long felt need in the art for effective viral drug delivery vehicles that can effectively evade the phagocytic cell uptake of the immune system, thereby resulting in a reduction of an immune response. The present invention meets this need.